This invention relates to large dynamoelectric machines and stator wedging systems and in particular to an improved dovetail wedge for retaining conductor bars in stator core slots.
Large dynamoelectric machines such as electrical generators employ a laminated stator core for transmitting induced voltages to the generator terminals through stator conductor bars. The cores are usually made by assembling already-slotted punchings or laminations into an annular housing for later containing the generator rotor. The slotted punchings, when assembled, define axially-extending radial slots which terminate at the radially inner-circumference of the stator annulus. The stator bars, or conductors, are laid in the radial slots and a wedging system is used to hold the bars in place against electromagnetic forces present when the machine is operating. If the wedging system is not effective, conductor insulation may be damaged in the ensuing vibration, ultimately leading to a forced outage of the generator.
U.S. patent application Ser. No. 307,007 filed Sept. 30, 1981 to inventor Katsekas and assigned to the assignee of the present invention shows a new and improved wedging system for a dynamoelectric machine stator core. The parts to the wedging system include filler strips, pressure wedges, curling wedges and dovetail wedges. This new wedging system increases the radial force applied against the conductor bars thereby providing a higher performance wedging system than was heretofore available in the industry. A machine under operating conditions might produce bar forces on the order of from 5 to 20 lbs/in. whereas the new wedging system as described in the aforesaid patent application generates radial forces on the order of from 30 to 100 lbs/in. against the bar forces. In other words, the new wedging system provides an antibar force of from 5 to 6 times the magnitude of the bar forces encountered in the operation of the machine. For this reason, the present invention has been developed to ensure the performance and longevity of the stator wedge.
U.S. Pat. No. 4,200,818 to Ruffing et al. issued Apr. 29, 1980 shows a dovetail wedge which is comprised of two distinct pieces; namely, a glass fiber core surrounded on at least two sides with a facing layer of mat fiber; i.e., Kevlar, both parts being resin impregnated. The mat fiber provides a protective and lubricating interface between the stator core laminations and the stator slot wedges. The stator core laminations each have an electrically insulating coating which if interrupted or otherwise disturbed could lead to short circuits and localized overheating within the dynamoelectric machine.
One requirement of a stator wedge is that it be able to withstand radial forces without splitting apart. According to the Ruffing et al. patent, the typical phenolic resin Kraft paper slot wedge has a strength on the order of 1,500 lbs/in. of length. The wedge made in accordance with Ruffing et al. teaching has a strength of 3,510 lbs/in. of length. The wedge yet to be described and made in accordance with the present invention is an improvement over the prior art in that it has a strength of 9,100 lbs/in. length which represents an improvement over the prior art on the order of 260 percent.